1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system which controls the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio in a feedback manner based on outputs from a plurality of exhaust gas component concentration sensors arranged in the exhaust passage of the engine.
2. Prior Art
There are conventionally known air-fuel ratio control systems which are applied to an internal combustion engine which is provided with first and second exhaust gas-purifying catalytic converters serially arranged in the exhaust system at respective upstream and downstream locations, and first and second exhaust gas component concentration sensors arranged, respectively, at locations upstream and downstream of the first catalytic converter, and wherein feedback control of the air-fuel ratio of an air-fuel mixture to be supplied to the engine is carried out, based on outputs from these exhaust gas component concentration sensors to thereby improve exhaust emission characteristics of the engine, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 5-321651 (hereinafter referred to as "Prior Art 1") and Japanese Laid-Open Patent Publication (Kokai) No. 2-67443 (hereinafter referred to as "Prior Art 2").
According to Prior Art 1, the second exhaust gas component concentration sensor is arranged at a location intermediate the two catalytic converters in order to secure required responsiveness of the feedback control, which, however, results in incapability of monitoring final components present in exhaust gases downstream of the second catalytic converter, i.e. exhaust gases emitted from the engine into the air. On the other hand, according to Prior Art 2, the second exhaust gas component concentration sensor is arranged downstream of the second catalytic converter, and therefore final components present in exhaust gases emitted from the engine can be monitored. However, Prior Art 2 suffers from degraded responsiveness of the feedback control. Therefore, the prior art has room for further improvement in the purification of exhaust gases emitted from the engine.